The present invention relates to a digital transmission system for time division multiplexed transmission of a plurality of digitized telephone signals or the like signals and, more particularly to a digital transmission system of this kind capable of frequency band compression with improved transmission performance.
Among various approaches to the frequency band compression for the multiplexed transmission of telephone signals, the so-called "time assignment speech interpolation (TASI) system" and, particularly "digital speech interpolation (DSI) system" have been in extensive use. Briefly stated, these conventional systems are based on the fact that an average telephone conversation has as much as 60 percent of speech inactivity time, in which the voice is absent. Instead of providing the transmission channels equal in number to the telephone signals to be transmitted, the conventional systems provide a much smaller number of channels, which are shared by the telephone signals in such a manner that one of the signals is inserted into the speech inactivity period of another. For details of the conventional systems, particularly of the DSI system, reference is made to U.S. Pat. No. 4,002,841 issued on Jan. 11, 1977 to Bell Telephone Laboratories Inc., and a paper titled "Ditigal Speech Interpolation" by S. J. Campanella, published in COMSAT Technical Review, Vol. 6, No. 1 (Spring issue, 1976), pp 127-158.
For the above outlined conventional systems to achieve the desired results, the presence and absence of the speech signal must be determined on each of the incoming signal paths. This is usually performed by a speech detector, which determines the presence of the signals with respect to a reference significant signal level. However, a slight time lag is unavoidably involved in the speech signal detection at the speech detector, resulting in the mutilation of conversation or speech clipping at the start of each segment of the speech signal. In order to overcome this problem, the conventional DSI system employs, for each of the signal paths, a delay circuit which brings the incoming signal into synchronism with the speech detector output. If the signal becomes out of coincidence with the detector output due to additional delay caused by long transmission paths, the talker's echo disturbs the conversation. Need arises accordingly for echo suppressors or cancellers, increasing the cost per transmission channel. This difficulty will be seen from the description given in U.S. Pat. No. 4,059,730 issued on Nov. 22, 1977 to Bell Telephone Laboratories, Inc. and U.S. Pat. No. 3,644,680 issued on Feb. 22, 1972 to Fujitsu Limited. The same problem applies to the transmission of other similar digital signals of comparable frequency bandwidth involving inactivity time periods.